Silicones have been used extensively for a variety of uses including release coatings for paper and paperboard. Other exemplary uses of silicone coatings on paper and paperboard include imparting improved slip, abrasion resistance and water repellency. Silicone coating compositions suitable for release coatings are well known in the art. The compositions are generally applied as an aqueous emulsion, a dispersion in an organic solvent, or in the compound form as a silicone polymer. The compound form uses no carrier and is typically referred to as a solventless composition containing 100% actives.
Silicone coatings are known to have very good release properties for use with pressure sensitive adhesives. In such uses the silicone coated paper may be used as a release backing for pressure sensitive adhesive tapes and the like. The release backing is easily removed to expose the adhesive surface. Silicone coating compositions are particularly desirable for release coatings because they are non-toxic and are relatively inert to the pressure sensitive adhesives. Silicone coating composition further have the advantage of being able to be formed into compositions which have good flow properties so as to be easily applied by conventional coating equipment.
Release coatings must generally be easy to apply to the substrate and spread uniformly. The release coatings must provide a low release force from the adhesive and not adversely affect the subsequent adhesion of the pressure sensitive adhesive tape. Release force is generally intended to refer to the amount of peel force required to separate the adhesive coated tape from the release layer without transfer from the adhesive tape. The most efficient release coatings form uniform coatings, are readily cured and have a high gloss surface.
Silicone coating compositions used for release coating are typically applied to a paper, and cured in an appropriate manner to form the release backing. The silicone component may be, for example, a silanol terminated polydimethylsiloxane, a hydrogen terminated polydimethyl siloxane, or an organomodified siloxane having a terminal functional group such as a reactive hydroxyl or a reactive vinyl group. Crosslinking of the silicone is typically initiated by U.V. radiation, a catalyst, heat, or electron beam radiation. One example of a curable silicone composition may include a silanol terminated organopolysiloxane, a hydride polysiloxane crosslinking agent and a metal salt of a carboxylic acid as a catalyst. Other additives may be included in the compositions such as thickeners or emulsifying agents. The curing time for the silicone should be sufficiently fast to cure properly within the time constraints of the process and without requiring a subsequent curing step.
One example of a silicone release coating composition is disclosed in U.S. Pat. No. 4,190,688 where a silanol terminated, vinyl modified silicone is applied to paper as an emulsion. The catalyst disclosed is a platinum catalyst and a tin salt of a carboxylic acid. The silicone is curable at room temperature or at elevated temperatures.
Another example of an emulsion silicone coating composition is disclosed in U.S. Pat. No. 4,476,241. This composition includes a silanol terminated organomodified silicone and a tin dineodeconate catalyst. The coating composition is heated to an elevated temperature to promote crosslinking of the silicone. The silicone emulsion is reported to provide release coatings with a long bath life, stability, low odor and reactivity over a wide temperature range.
U.S. Pat. No. 4,201,808 relates to a solventless radiation curable silicone release coating containing an acryloxy or methacryloxy containing organomodified silicone, a polyacrylyl crosslinking agent and a photosensitive initiator. The composition is applied to a substrate and cured by U.V. radiation.
The general trend in the industry is to use reactive silicone aqueous emulsions since they are easy to form and can be easily applied using conventional coating equipment. The amount of the silicone applied to the substrate can be controlled by adjusting the concentration of the silicone in the emulsion. Solvent based silicone compositions can also be applied by conventional coating apparatus such as, for example, gravure, meyer rod or air knife coaters. These coating compositions do not generally require thickeners or other viscosity adjusting agents to produce a suitable coating. The viscosity of the silicone is generally sufficient to provide adequate coat weight and uniform application.
Solvent based silicone coating compositions can be used effectively to form uniform release coatings on substrates. These silicones, however, have several disadvantages which limited their use. The organic solvents used as the carrier for the silicones are generally volatile and create health related risks to workers and the environment. The solvents used in the silicone compositions must be removed by evaporation and, therefore, must be recovered to prevent escape of the vapors into the atmosphere. The solvent recovery equipment needed to prevent vapor loss to the environment requires a large capital expenditure and results in high energy operating costs. In addition, solvent based silicone compositions often require large curing ovens to properly cure the silicone. Solvent based coating compositions may require additional equipment to handle the solvents. The viscosity of solvent based silicone coating compositions can be adjusted by diluting the silicone with an appropriate solvent, such as for example, benzene, toluene or xylene. This dilution, however, increases the solvent handling and recovery expenses.
Solventless silicone coating compositions may be applied by several methods, such as a differential offset gravure coater. These silicone coating compositions can be used to produce good release coatings provided the coating is applied uniformly. Since the silicone composition is applied in the 100% actives form, care must be exercised to apply the coating uniformly and in very thin films to maintain an economically acceptable coat weight. To improve handling and coating rheology, these compositions may be diluted with a suitable solvent.
Aqueous silicone emulsions have the advantage of being easily formed and can be applied to a substrate using conventional apparatus, such as size press, meyer rod or gravure roll. Aqueous emulsions can be easily diluted with water to achieve the appropriate actives levels. Commercially available silicone emulsions are generally supplied at 30% to 50% actives. The silicone emulsion is usually diluted to about 8% to 20% actives to insure uniform application of the silicone on the paper.
A disadvantage of aqueous silicone emulsions for producing release coatings is the tendency of the silicone to penetrate the paper. Generally it is more desirable to increase the amount of silicone on the surface of the substrate, which is typically referred to as holdout. To improve holdout of the silicone, thickeners may be added to increase the viscosity and water binding capacity and to increase the amounts of silicone that can be applied to the substrate under normal running conditions. Thickeners typically used include modified cellulose derivatives such as carboxymethyl cellulose and modified gums.
Thickeners may be used to improve holdout of the silicone from the substrate, but the thickeners often impart undesirable properties to the coated sheet. Some thickeners may produce a decreased gloss to the cured silicone coating, reduce the adhesive release from the silicone coating, and inhibit curing of the silicone. As a result of these disadvantages, aqueous silicone emulsion coatings usually produce release coatings that do not provide the high gloss and adhesive release comparable with solvent based and solventless silicone coating compositions at the same coat weight. It is often necessary to apply a higher coat weight of an aqueous emulsions to achieve similar gloss and release properties of solventless and solvent based compositions.
Without the use of a suitable thickener in the aqueous silicone emulsion, the penetration of silicone into the substrate is sufficiently high to reduce the release and gloss properties. The coat weight of the silicone can be increased to accommodate for the penetration. The increased cost associated with the increased coat weight has prevented this from being a viable alternative to the use of thickeners in spite of the inherent disadvantages of using thickeners and viscosity adjusting agents.
The general trend is to use alternatives to the solvent based silicone compositions and the volatile organic solvents used in these compositions. Solventless compositions do not provide the versatility of application of the silicone and are more difficult to apply economically and uniformly. The performance deficiencies of aqueous silicone emulsions has, however, limited their use and hindered conversions from the use of organic solvents.
There is still a need for a silicone coating composition which can be used without the volatile organic solvents and is able to provide uniform silicone coatings with sufficient silicone holdout. The present invention is directed to an aqueous silicone emulsion composition which is able to provide good release properties and is easy to apply without excessive coat weight. The composition and method of the invention produce silicone release coatings exhibiting release and gloss properties which have heretofore only been obtainable from solvent based or solventless silicone compositions.